Dolomite

Hardness: 3.5 to 4

The mineral dolomite is composed of calcium magnesium carbonate CaMg(CO3)2 and is closely related to calcite, the main component of limestone. In large masses, the mineral forms the rock called dolomite. It may be white, gray, greenish gray, brown, or pink, and it has a glassy to pearly luster. It occurs in coarse- to fine-grained granular masses and in crystals. Dolomite is slightly harder than calcite, although it can easily be scratched by a knife. It will not fizz readily in dilute solutions of cold hydrochloric acid unless first ground to a powder.